The science case for ELTs: personal impressions
from the first workshop.

1. Introduction

About 40 people gathered together at the Royal Observatory Edinburgh for
two days (Sept 25-26, 2000) to discuss a possible science case for extremely
large telescopes (30-100m class). The matter was subdivided into three
topics/panels:

I took part in the discussions in panel (a). Here, I briefly summarise
my personal impressions, following the joint discussion of the individual
panels.

2. Highlights of the Joint Discussion

(a) stars and planets

in the optical: imaging of the "faces of nearby stars" (stellar disks)

in the near-IR: imaging of dust formation in red giants ("dust movie")

in the mid-IR : witnessing "Jupiters in the making" (resolving 2AU tidal
gaps in circumstellar/protostellar disks at distances of the order of 150
pc at 10 microns). This science needs 100m telescopes rather than 30m.

(b) galaxies and stellar populations

study IMF variations in the Local Group (see hydrogen burning limit in
M31/M33)

3. Conclusions

No hard conclusions were given, but some clues emerged. The stars/planets
domain would probably benefit most from the imaging capability of OWL at
very high angular resolution (2mas at 1 micron for 100m telescope diameter,
i.e. 1 AU at the distance of the Orion star formation complex). On the
other hand, cosmology would benefit most from the vastly increased collecting
area compared to the VLT and Keck; by 2010-2015 there will be a need for
follow-up spectroscopy of NGST imaging.

As for galaxies and stellar populations, the benefits did not appear
to be so overwhelming, as the new thresholds depended only on optical observations,
and left the audience with mixed feelings.

Building gigantic telescopes like OWL will make sense only, if the problems
of multi-conjugate adaptive optics (MCAO) can be solved. The optimism expressed
at the meeting, based on computer simulations, must be backed up by real
systems. It was also noted that lack of wind would be a primary site selection
criterion rather than the best seeing conditions. PS. With hindsight, it
is unclear to me whether a fully filled circular aperture is the best solution
for the new science listed above. For example, with the same collecting
area one could also build a diluted aperture telescope (non-redundant baseline
array) in which some baselines can be larger by a factor of two over the
canonical circular diameter, thus increasing the diffraction limit and
angular resolution correspondingly.